1. Field of the Invention
This invention relates to systems for tempering glass sheets as well as to furnaces utilized in such systems.
2. Description of the Prior Art
Glass sheets are manufactured by first drawing the sheets in a molten form from a bath of molten glass. The molten glass sheets, which are usually of a continuous nature but may also be of a discrete length, are then conveyed through an annealing furnace having a decreasing temperature gradient in a direction along which the glass sheets are conveyed. The decreasing temperature gradient causes the glass to be cooled at a slow rate which prevents the buildup of compressive forces in the glass. Annealed glass may be cut and drilled so as to have the size and configuration desired. After being cut to size and drilled as required, the annealed glass may also be tempered by a heating and sudden cooling process which gives the glass sheets high compressive forces at their surfaces. Tempered glass sheets are less susceptible to breakage, and break into small pieces that are dull and relatively harmless instead of into large, sharp pieces as is the case with untempered glass.
To perform the tampering of glass sheets, systems have been developed in the past utilizing horizontal roller conveyors that convey the glass sheets horizontally first through a furnace and then to a quench unit where the sudden cooling is performed. One such glass tempering furnace is disclosed by the U.S. Pat. No. 3,806,312, of McMaster et al. issued Apr. 23, 1974, and other glass furnaces are shown by the reference cited therein. When being conveyed through a tempering furnace by a horizontal roller conveyor, glass sheets must be moved fast enough so as not to sag between the conveyor rollers. The glass must be moved faster to prevent the sagging when there is a greater spacing between the conveyor rollers are opposed to a smaller spacing. When the roller spacing is on the order of 4 to 6 inches, the glass movement is normally on the order of 45 to 70 feet per minute in order to prevent sagging.
When the hot glass sheets being tempered move from the furnace into the quench unit, warpage and breakage tend to occur because the leading edge of the glass is being cooled and caused to shrink while the trailing end is still hot and in its thermally expanded condition. This tendency is more severe the slower the travel and the wider the glass. Consequently, very wide glass, i.e. 80 inches or so, must be moved into the quench much more rapidly than narrower sheets when conveyed along a continuous conveyor from the furnace to the quench unit. However, this high rate of glass movement necessarily requires the furnace length to be very long, so long in fact that the furnace could not be economically justified in view of the relatively small market for very wide glass.
U.S. patents that are similar to the present invention in some manner are described below, but the differences between these patents and the invention herein disclosed will be apparent.
The U.S. Pats. Nos. 1,856,668, 1,856,669 and 1,879,998 of Julius Sylvester disclose glass annealing systems that incorporate horizontal roller conveyors. These conveyors carry sheets of glass having discrete lengths horizontally through elongated annealing furnaces that have a decreasing temperature gradient in a direction along which the glass is conveyed. The conveyor rollers are alternately rotated in a forward and rearward rotation so that the glass is conveyed along the decreasing temperature gradient in a "two steps forward and one step backward"manner to thereby provide annealing of the glass. Such a furnace requires a large number of reversals of the direction of roller rotation in order to provide the proper treatment of the glass.
The U.S. Pat. No. 3,447,788 of June R. Bornor discloses a furnace having a horizontal roller conveyor that reciprocates a workpiece to be heated a slight amount in order to avoid the concentration of heat on the same areas of the workpiece. The purpose of this reciprocation is to insure uniform heating of the workpiece within the furnace.